Since 1999, a new disease which causes symptoms described as “yellowing of melon plants” was reported to cause damage in north-eastern Brazil, which is the region where more than 90% of the Brazilian melon production takes place. Symptoms are leaf mottling and yellowing and are mainly seen on older leaves (Nagata et al. 2003, Plant Pathology 52, 797). The virus causing this disease was tentatively named Melon yellowing-associated Virus (MYaV) (Nagata et al., 2003, supra and Nagata et al., 2005, Arch. Virology Vol. 150(2):379-87). In 2007 serological detection (using a polyclonal anti-bodies developed for MYaV detection, (see Avila et al. 2008 Trop. Plant Pathol. v. 33 n. 3 Brasilia May/June 2008) revealed that a large percentage of symptomatic melon plants were indeed infected with MYaV (Lima et al. Hortic. Bras. vol. 27 no. 4 Brasilia October/December 2009). The worst affected region was in the state Rio Grande do Norte, in Mossoro, with 96.3% of melons being infected. Interestingly, virus concentrations were higher in extracts prepared from stems of symptomatic plants than from leaves.
The typical symptoms of the disease appear as leaf mottling and yellowing, mainly of older leaves, similar to a nutritional disorder (see Nagata et al, 2003, supra and FIG. 1 of Nagata et al. 2010, Journal of General Plant Pathology Volume 76, No. 4, page 268-272). In infected leaf tissue showing yellowing symptoms filamentous virus particles of 600-700 nm length can be seen by electron microscopy.
The virus found in plants with the yellowing disease symptoms is transmitted from melon to melon plants by whiteflies (Bemisia tabaci biotype B). Also grafting can be used to transmit the virus to other melon plants or to Cucumis anguria (West Indian gherkin). By electron microscopy, long, filamentous Carlavirus-type particles and inclusion bodies were seen in infected leaves, which suggested the presence of a virus of the genus Carlavirus (Nagata et al. 2003, Plant Pathol 52:797). Nagata et al. 2005 (supra) sequenced two genes, the coat protein (ORF-A) and one more open reading frame (ORF-B), see GenBank Accession number AY373028. As Cowpea mild mottle virus (CPMMV) was the only carlavirus species known to be transmitted by whiteflies, genetic and serological properties of MYaV were expected to be similar to CPMMV. However, MYaV did not cross-react in a dot-immunobinding assay to antibody of CPMMV (Nagata et al. 2003, supra), and genomic sequence data showed that the coat protein (CP) of CPMMV was not closely related to that of MYaV (Nagata et al. 2005, supra).
Initially it was unclear whether to include MYaV within the Carlavirus genus or if it should be a new genus in the family Flexiviridae (Nagata et al. 2005, supra). However, in a recent study (Nagata et al. 2010, supra), an estimated 40% (ca. 3.1 kb) of the MYaV genome was cloned and sequenced and based on these data the authors suggest that the virus is indeed a new species within the genus Carlavirus and they suggest to change the name of this virus to Mellon Yellowing Virus (MYV). The 3.1 kb sequence contained 5 open reading frames (ORFs), encoding three Triple Gene Block proteins (TGB1, TGB2 and TGB3), the coat protein (CP) and putative nucleic acid binding protein (NABP), see GenBank Accession number AB510477. The coat protein (CP) sequence in this study had 93% sequence identity to the sequence of ORF-A (AY373028).
As no plants with resistance against the virus are available, one strategy developed to limit MYaV infection is to cover the whole field with spunbond nonwoven fabric layer from germination until flowering, to prevent whitefly transmission of the virus. However, plants became sensitive to leaf miners (Liriomisa spp.), which became widespread and heavily damaged fruit production (Nagata et al. 2010, supra).
It is an object of the invention to provide MYaV resistance sources and a genetic region comprising the resistance locus or a part thereof, which confer resistance against MYaV. It is a further object of the invention to provide cultivated melon plants (Cucumis melo L.) and cells, tissues, fruits and other parts of such plants comprising in their genome a MYaV resistance-conferring locus (or a resistance-conferring part thereof), either in homozygous or heterozygous form, whereby the melon plants are resistant against MYaV. Also seeds from which MYaV resistant melon plants can be grown are an embodiment of the invention.
In a further aspect molecular markers are provided, which can be used to detect the presence of and/or to transfer the MYaV resistance-conferring locus, or a resistance-conferring part thereof, in/into plants or plant cells of Cucumis melo L. One or more of the markers can, thus, for example be used to transfer the resistance locus, or a resistance-conferring part thereof, into melon plants which are susceptible to MYaV. In one embodiment the resistance locus, or resistance-conferring part thereof, is the locus on chromosome 6 as found in seeds deposited under accession number NCIMB 41966 or NCIMB 41969. In a further embodiment the resistance locus or resistance-conferring part thereof is the locus on chromosome 6, or a resistance-conferring part thereof, as found in other wild melon plants or wild relatives of melon.
One or more of the markers linked to, or associated with, the MYaV resistance locus, or resistance conferring part thereof, can also be used to identify new MYaV-resistance sources, such as other wild accessions of Cucumis melo or wild relatives of melon comprising an MYaV-resistance locus on chromosome 6 and for transferring (introgressing) the resistance locus, or a MYaV-resistance conferring part thereof, from such accessions into cultivated melon plants. The MYaV resistance conferring quantitative trait locus (QTL) on chromosome 6 (equivalent to ICuGI Linkage Group VI, or LG VI) was named MYaV6.1.
EP1962578B1 describes a CYSDV resistance QTL of PI313970 on a linkage group which is therein arbitrarily designated as LG6 and claims melon plants comprising an introgression from PI313970, which introgression comprises a CYSDV resistance QTL linked to at least one marker located on the chromosome equivalent to linkage group (LG) 6 of melon accession PI313970. It is noted that the in EP1962578B1 arbitrarily named LG6 is ICuGI LG VI of melon, but corresponds to ICuGI Linkage Group V (LG V). In one aspect the plant of the invention i.e. a cultivated Cucumis melo plant comprising resistance against Melon Yellowing associated Virus (MYaV) wherein said resistance is conferred by an introgression fragment on chromosome 6 in homozygous or heterozygous form and wherein said introgression fragment is from a wild plant of the species Cucumis melo, does not comprise the CYSDV resistance QTL as described in EP1962578B1. In another aspect the plant of the invention i.e. a cultivated Cucumis melo plant comprising resistance against Melon Yellowing associated Virus (MYaV) wherein said resistance is conferred by an introgression fragment on chromosome 6 in homozygous or heterozygous form and wherein said introgression fragment is from a wild plant of the species Cucumis melo does not comprise the markers E11/M49-239, as defined in paragraph [0037] of EP 1962578 B1. In yet another aspect the plant of the invention does not comprise the markers E11/M54-156, E14/M54-152, E14/M51-210, E14/M51-083, E11/M49-239, E11/M54-169, E14/M50-262, E11/M57-278, E11/M54-163 and/or E11/M49-072 as defined in paragraph [0040] of EP1962578 B1. In still another embodiment the plant of the invention does not comprise the markers E11/M54-156, E14/M54-152, E14/M51-210, E14/M51-083, E11/M49-239, E11/M64-169, E14/M60-262, E11/M67-278, E11/M64-163 and/or E11/M49-072 as defined in paragraph [0013] of EP 1962578 B1. The cited passages of EP1962578B1 are enclosed herein by reference. In still another aspect the plants of the invention, i.e. a cultivated Cucumis melo plant comprising resistance against Melon Yellowing associated Virus (MYaV) wherein said resistance is conferred by an introgression fragment on chromosome 6 in homozygous or heterozygous form and wherein said introgression fragment is from a wild plant of the species Cucumis melo, does not have a CYSDV phenotype (i.e. is not resistant to CYSCV).